1. Field of the Invention
Example aspects described herein relate generally to optical communications systems employing dense wavelength division multiplexing (DWDM), and more particularly, to methods, apparatuses, systems, and computer programs that employ a reconfigurable optical add/drop multiplexer (ROADM) network element that is compatible with both C-band and L-band optical signals.
2. Description of the Related Art
Conventional dense wavelength division multiplexing (DWDM) optical networks achieve 10 gigabit per second (Gb/s) transmission in the C-band (i.e., the band of wavelengths from 1525 nanometers to 1560 nanometers) by using a modulation scheme called on-off keying (OOK). Nowadays, with increasing data usage, it would be desirable to achieve optical network bitrates higher than 10 Gb/s, such as, for example a bitrate of 100 Gb/s. However, achieving such higher bitrates using conventional networks is problematic due to certain optical limitations inherent to 10 Gb/s OOK C-band transmission. The bitrate of conventional C-band networks is limited by factors inherent to OOK, as well as by the inherent characteristic maximum bandwidth associated with erbium-doped fiber amplifiers (EDFAs), which are deployed in many C-band optical networks.
To achieve bitrates higher than 10 Gb/s (e.g., 100 Gb/s) while avoiding the above limitations, some networks have been developed that employ neither OOK modulation nor EDFAs. These networks employ a modulation scheme called polarization mode-quadrature phase shift keying (PM-QPSK) in the L-band wavelength band (i.e., the band of wavelengths ranging from 1565 nanometers to 1625 nanometers), without the use of EDFAs, and thereby avoid the above limitations of OOK and EDFAs.
There are several advantages to using a modulation scheme such as PM-QPSK. For example, the use of PM-QPSK enables optical signal transmission within the 50 GHz band at information rates of 100 Gb/s, better utilizing available bandwidth and enabling the use of less costly components (e.g., components designed to operate at frequencies less than 100 GHz). Also, PM-QPSK networks are much less sensitive to chromatic dispersion and polarization mode dispersion than other common modulation schemes (e.g., OOK).
However, there are unique challenges associated with using PM-QPSK in conventional C-band ROADM optical networks. For example, the optical signal quality (e.g., optical signal to noise ratio or OSNR) of PM-QPSK signals is degraded by dispersion compensating modules (DCMs), which are deployed in conventional C-band optical networks to correct for chromatic and/or polarization dispersion. Therefore, communicating 100 Gb/s signals in existing C-band ROADM networks through the use of PM-QPSK in the L-band is problematic because such C-band networks commonly include signal quality degrading DCMs.
One previous approach to solve this problem involved mixing 100 Gb/s networks in with current ROADM traffic. However, this approach requires additional costly regenerating transponders to compensate for the degradation of the L-band signals caused by the DCMs used in current networks. That is, at periodic points throughout the network, the regenerating transponders regenerate the L-band signals before the signals are degraded to a point of being unrecoverable. In most cases, regenerating transponders are implemented using two similar transmission cards at an ingress and an egress point of the network. Some large and/or long distance networks would require many regenerating transponders, which would make updating such networks with regenerating transponders quite costly.
Another previous approach to solving this problem involved installing additional ROADM modules along with additional WDM optical input/output ports (referred to as “ROADM degrees”) at ROADM network elements to support 100 Gb/s traffic throughout the network. However, the number of ROADM degrees allowed at a single network element is limited (for example, by limitations inherent to one or more components (e.g., switches) of the network element), typically to four or eight degrees. If a service provider is using an eight-degree ROADM, it is not uncommon for five of those degrees to be occupied for existing C-band traffic, leaving only three available degrees with which to install 100 Gb/s links, which limits the flexibility in upgrading the network architecture.
Rather than developing a new large-scale optical network infrastructure for high-bitrate communication using PM-QPSK signals in the L-band, which could be quite costly, it would be desirable to integrate L-band PM-QPSK signals into existing C-band optical network infrastructure, to the extent possible.
However, without any device modifications, employing PM-QPSK signals in the L-band in existing C-band optical networks is problematic, because, as mentioned above, some techniques employed in optimizing signals in existing C-band networks (e.g., correcting certain characteristics of current OOK modulation and/or EDFAs) cause signal degradation of PM-QPSK signals in the L-band, as well as other problems.
Therefore, it would be useful to modify, in a cost-effective manner, conventional C-band optical networks to be compatible with both C-band and L-band optical signals.